In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of processing the message signal into a form suitable for transmission over the channel. This processing of the message signal is referred to as modulation. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to process the received signal so as to produce an estimate of the original message signal. This processing of the received signal is referred to as demodulation.
Two types of two-way communication channels exist, namely, point-to point channels and point-to-multipoint channels. Examples of point-to-point channels include wirelines (e.g., local telephone transmission), microwave links, and optical fibers. In contrast, point-to-multipoint channels provide a capability where many receiving stations may be reached simultaneously from a single transmitter (e.g. cellular radio telephone communication systems). These point-to-multipoint systems are also termed Multiple Address Systems (MAS).
In any communication system, there are two primary communication resources to be employed, namely, average transmitted power and channel bandwidth. The average transmitted power is the average power of the transmitted signal. The channel bandwidth defines the range of frequencies that the channel can handle for the transmission of signals with satisfactory fidelity. A general system design objective is to use these two resources as efficiently as possible. In most channels, one resource may be considered more important than the other. Hence, we may also classify communication channels as power-limited or band-limited. For example, the telephone circuit is a typical band-limited channel, whereas a deep-space communication link or a satellite channel is typically power-limited.
The transmitted power is important because, for a receiver of prescribed noise figure, it determines the allowable separation between the transmitter and receiver. In other words, for a receiver of prescribed noise figure and a prescribed distance between it and the transmitter, the available transmitted power determines the signal-to-noise ratio at the receiver input. This, subsequently, determines the noise performance of the receiver. Unless performance exceeds a certain design level, the transmission of message signals over the channel is not considered to be satisfactory.
Additionally, channel bandwidth is important because, for a prescribed band of frequencies characterizing a message signal, the channel bandwidth determines the number of such message signals that can be multiplexed over the channel. In other words, for a prescribed number of independent message signals that have to share a common channel, the channel bandwidth determines the band of frequencies that may be allotted to the transmission of each message signal without discernible distortion.
Analog and digital transmission methods are used to transmit a message signal over a communication channel. The use of digital methods offers several operational advantages over analog methods, including but not limited to: increased immunity to channel noise and interference, flexible operation of the system, common format for the transmission of different kinds of message signals, improved security of communication through the use of encryption, and increased capacity.
These advantages are attained at the cost of increased system complexity. However, through the use of very large-scale integration (VLSI) technology, a cost-effective way of building the hardware has been developed.
One digital transmission method that may be used for the transmission of message signals over a communication channel is pulse-code modulation (PCM). In PCM, the message signal is sampled, quantized, and then encoded. The sampling operation permits representation of the message signal by a sequence of samples taken at uniformly-spaced instants of time. Quantization trims the amplitude of each sample to the nearest value selected from a finite set of representation levels. The combination of sampling and quantization permits the use of a code (e.g., binary code) for the transmission of a message signal. Other forms of digital transmission use similar methods to transmit message signals over a communication channel.
When message signals are digitally transmitted over a band-limited channel, a form of interference known as intersymbol interference may result. The effect of intersymbol interference, if left uncontrolled, is to severely limit the rate at which digital data may be transmitted without error over the channel. The cure for controlling the effects of intersymbol interference may be controlled by carefully shaping the transmitted pulse representing a binary symbol 1 or 0.
Further, to transmit a message signal (either analog or digital) over a band-pass communication channel, the message signal must be manipulated into a form suitable for efficient transmission over the channel. Modification of the message signal is achieved by means of a process termed modulation. This process involves varying some parameter of a carrier wave in accordance with the message signal in such a way that the spectrum of the modulated wave matches the assigned channel bandwidth. Correspondingly, the receiver is required to recreate the original message signal from a degraded version of the transmitted signal after propagation through the channel. The re-creation is accomplished by using a process known as demodulation, which is the inverse of the modulation process used in the transmitter.
In addition to providing efficient transmission, there are other reasons for performing modulation. In particular, the use of modulation permits multiplexing, that is, the simultaneous transmission of signals from several message sources over a common channel. Also, modulation may be used to convert the message signal into a form less susceptible to noise and interference.
For multiplexed communication systems, the system typically consists of many remote units (i.e. subscriber units) which require active service over a communication channel for short or discrete intervals of time rather than continuous service on a communication channel at all times. Therefore, communication systems have been designed to incorporate the characteristic of communicating with many remote units for brief intervals of time on the same communication channel. These systems are termed multiple access communication systems.
One type of multiple access communication system is a frequency division multiple access (FDMA) system. In an FDMA system, the communication channel is divided into several narrow frequency bands. Individuals communication channel links are established between two communication units within one of these narrow frequency bands. These communication links are maintained for discrete amounts of time while the two communication units transmit and receive signals. During particular communication links between the two communication units, the communication system does not allow other communication units access to the narrow frequency band within the communication channel which is being utilized by the communication units in the particular communication link.
Another type of multiple access communication system is a time division multiple access (TDMA) system. In a TDMA system, the communication channel is divided into time slices of a time frame to allow communication links between two communication units to exist in the same communication channel simultaneously, but at different time slices. This is accomplished by assigning particular time slices of a time frame to a particular communication link and other time slices to other communication links. During these particular communication links between the two communication units, the communication system does not allow other communication units access to the time slice of the time frame within the communication channel which is being utilized by the communication units in the particular communication link.
Further, another type of multiple access communication system is a spread spectrum system. In a spread spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. Other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, also permit information to be transmitted in a bandwidth comparable to the bandwidth of the information itself. However, in a spread spectrum system, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz, and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. This is accomplished by modulating the signal to be transmitted with the information to be sent and with a wideband encoding signal.
Unlike FDMA and TDMA systems, in spread spectrum systems a signal may be transmitted in a channel in which the noise power is higher than the signal power. The modulation and demodulation of the message signal using spread spread spectrum techniques provides a signal-to-noise gain which enables the recovery of the message signal from a noisy communication channel. The greater the signal-to-noise ratio for a given system equates to: (1) the smaller the bandwidth required to transmit a message signal with a low rate of error or (2) the lower the average transmitted power required to transmit a message signal with a low rate of error over a given bandwidth.
Three general types of spread spectrum communication techniques exist, including:
The modulation of a carrier by a digital code sequence whose bit rate is much higher than the information signal bandwidth. Such systems are referred to as "direct sequence" modulated systems. PA1 Carrier frequency shifting in discrete increments in a pattern dictated by a code sequence. These systems are called "frequency hoppers". The transmitter jumps from frequency to frequency within some predetermined set; the order of frequency usage is determined by a code sequence. Similarly "time hopping" and "time-frequency hopping" have times of transmission which are regulated by a code sequence. PA1 Pulse-FM or "chirp" modulation in which a carrier is swept over a wide band during a given pulse interval.
Information (i.e. the message signal) can be embedded in the spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. This technique can be used in direct sequence and frequency hopping systems. It will be noted that the information being sent must be in a digital form prior to adding it to the spreading code because the combination of the spreading code, typically a binary code, involves module-2 addition. Alternatively, the information or message signal may be used to modulate a carrier before spreading it.
Thus, a spread spectrum system must have two properties: (1) the transmitted bandwidth should be much greater than the bandwidth or rate of the information being sent and (2) some function other than the information being sent is employed to determine the resulting modulated channel bandwidth.
The essence of the spread spectrum communication involves the art of expanding the bandwidth of a signal, transmitting the expanded signal and recovering the desired signal by remapping the received spread spectrum into the original information bandwidth. Furthermore, in the process of carrying out this series of bandwidth trades, the purpose of spread spectrum techniques is to allow the system to deliver error-free information in a noisy signal environment.
Spread spectrum communication systems can be multiple access systems like FDMA and TDMA communication systems. One type of multiple access spread spectrum system is a code division multiple access (CDMA) system. In a CDMA system, communication between two communication units is accomplished by spreading each transmitted signal over the frequency band of the communication channel with a unique user spreading code. As a result, transmitted signals are in the same frequency band of the communication channel and are separated only by unique user spreading codes. Particular transmitted signals are retrieved from the communication channel by despreading a signal representative of the sum of signals in the communication channel with a user spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. A CDMA system may use direct sequence or frequency-hopping spreading techniques.
Each of these multiple access systems (i.e. FDMA, TDMA, and CDMA) can be utilized in cellular radio communication systems. In cellular radio communication systems, factors which limit the performance of system are distortion and noise. Typically, in propagating through a communication channel, a transmitted signal is distorted because of nonlinearities and imperfections in the frequency response of the communication channel. Other sources of degradation are noise and interference picked up by the signal during the course of transmission through the communication channel.
There are various sources of noise in a CDMA spread spectrum communication system. The sources of noise may be internal as well as external to the communication system. In CDMA, the majority of noise in the communication channel is from signals which are being transmitted with unique user spreading codes. These spread signals contribute to the overall noise in the communication channel.
To limit the noise in cellular communication systems as well as increase the capacity of the cellular communication systems, reuse the available but limited number of communication resources. To ensure the re-use of communication resources does not cause unacceptable noise in the communication channel, cell sites, which are allocated the same communication resources, are geographically separated. By having sufficient geographic separation, the noise in the communication channel is limited. However, the geographic separation needed to ensure an adequate signal-to-noise ratio (negligible noise in the communication channel) limits the capacity of the communication system because not all of the available communication resources may be used in each cell site.
To enhance the efficiency of communication resource re-use and to improve capacity of the cellular system, the cell sites can be divided into sectors, wherein each sector contains a percentage of the available communication resources. By having the cell sites divided into sectors, the required geographic separation may be reduced while maintaining an adequate signal-to-noise ratio. For example, U.S. Pat. No. 4,128,740, assigned to Motorola, Inc., discloses a four cell-six sector communication resource re-use pattern. As disclosed, each cell site is divided into six sectors and each sector contains approximately 1/24 of the available communication resources. For every four cell sites, the communication resource pattern is repeated. This communication resource re-use pattern may be further reduced to a one cell site re-use pattern as disclosed in pending U.S. application Ser. No. 07/459,624 which was filed Jan. 2, 1990 and is also assigned to Motorola Inc.
However, in a CDMA spread spectrum communication system, most of the noise in the communication channel in a particular cell site is from signals which are being transmitted with unique user spreading codes within that particular cell site. Thus, a one cell site reuse pattern is possible if the noise from cell sites surrounding the particular cell site is ignored. The total number of communication links that a particular cell site can simultaneously handle is related to the noise in the communication channel which is from sources external to the system. The use of this one site reuse pattern will limit the capacity of the communication system in this particular cell site to handle communication links, since noise from adjacent cell sites will be considered a part of the external noise for the particular cell site.
The present invention enhances the capability of spread-spectrum systems and, in particular, CDMA cellular radio-telephone systems to handle unequal channel loading of cells. In CDMA cellular radio-telephone systems, the "users" are in the same frequency band and separated only by unique spreading codes. The number of users that can use the same frequency band is limited by the noise interference level. The noise interference level is directly related to the interference level created by the users actively using the frequency band and nearby cell sites operating in the same frequency band. The present invention dynamically changes the noise level present in a frequency band and the communication channel load of individual cell sites to increase the capacity of the communication system.